1. Field of the Invention
The present invention relates to a photographing apparatus for photographing a tomographic image through use of optical interference, and more particularly, to a photographing apparatus including an interference optical system used for ophthalmologic care and a photographing method.
2. Description of the Related Art
In recent years, as an ophthalmologic apparatus using an optical device, various apparatuses have been used. For example, as an optical device for observing an eye, various apparatus such as an anterior ocular segment photographing apparatus, a fundus camera, and a scanning laser ophthalmoscope (SLO) have been used. Of those, an optical coherence tomography (OCT) apparatus (hereinafter referred to as “OCT apparatus”) is capable of acquiring a tomographic image of a sample with high resolution, and is becoming an apparatus necessary, as an ophthalmologic apparatus, for an out-patient clinic specializing in a retina.
The OCT apparatus irradiates a sample with low coherent light and disperses reflected light from the sample with an interference system to analyze and measure the sample with high sensitivity. Further, the OCT apparatus can acquire a tomographic image with high resolution by scanning the sample with the low coherent light. Therefore, the OCT apparatus can also photograph a tomographic image of a retina in a fundus of an eye to be inspected with high resolution, and hence, is being used widely in ophthalmologic diagnosis of a retina.
The OCT apparatus combines return light from an eye to be inspected, obtained by irradiating the eye to be inspected with low coherent light called measuring light through a scanning unit, with reference light corresponding to the measuring light, and acquires the above-mentioned tomographic image based on the obtained combined light. In this case, a position corresponding to an optical path length of the reference light in an optical path of the measuring light is referred to as a coherence gate. It is known that a tomographic image with higher resolution can be acquired as an object is closer to the coherence gate.
Japanese Patent Application Laid-Open No. 2008-154939 discloses a method of adjusting a signal-noise ratio of a one-dimensional sensor for analyzing combined light to a threshold value or more so as to adjust a coherence gate, for the purpose of acquiring a tomographic image of a retina with preferred resolution.
However, according to the method disclosed in Japanese Patent Application Laid-Open No. 2008-154939, a signal-noise ratio is used, and hence, it is difficult to determine an actual position of a coherence gate with respect to a retina. Therefore, in the case of acquiring a tomographic image with high resolution, there is a possibility that resolution at a position intended to be diagnosed may be degraded. Further, for example, in the case where cataract is developed in an eye to be inspected, and a signal-noise ratio cannot be obtained sufficiently, it is considered that it may be difficult to perform the above-mentioned control itself.
Further, in the conventional OCT apparatus, a tomographic image is photographed while being observed with a monitor. At this time, only a tomographic image which is being displayed is observed, and hence, it is difficult to perform an operation itself of bringing a coherence gate close to a retina.